1. Field of the Invention
The present invention relates generally to the field of optical communications and more particularly to an interference-based method and apparatus for wavelength slicing for use in dense wavelength division multiplexing (DWDM) applications.
2. Background Art
Optical communications is an active area of new technology and is crucial to the development and progress of several important technologies, e.g., Internet and related new technologies. A key technology that enables higher data transmission rate is the dense wavelength division multiplexing (DWDM) technology. In the DWDM technology, optical signals generated from different sources operating at predetermined, dense-spaced center wavelengths are first combined to form a single optical output. This single optical output is then transmitted, frequently amplified during transmission, through an optical fiber. The single optical output is then de-multiplexed, a process to separate individual data channels and each channel is then directed to its own destinations. In the DWDM technology, each data channel is assigned to a center frequency and the spacing between any two adjacent channels is a constant (e.g., 200 GHz or 100 GHz). It is also understood that all channels are given frequency windows with identical width. The width of these windows is kept great enough to pass information associated with these data channels and at the same time as narrow as possible to prevent cross-talking between different data channels. It is generally understood that the narrower the frequency spacing between different data channels, the greater the transmission capacity a DWDM system will have.
Several multiplexing and de-multiplexing devices are essential to the operation of a DWDM system. FIG. 1A is a diagram illustrating the operation of a group of devices known as optical filters. An optical filter has the function of separating signals within a predetermined frequency window from the input spectrum. In a DWDM system, to de-multiplex composite data, an optical filter is employed to separate signals associated with a particular data channel as depicted in FIG. 1A. Because each channel requires a specific filter, a DWDM de-multiplexer will require n optical filters in cascade in order to separate all of n channels into separate outputs. Using these filter cascades in the reverse direction will enable the construction of a multiplexer where individual signal channels with different center wavelengths can be combined together to form a single composed optical output signal. There are several types of optical filters and brief descriptions are provided for two types of commonly available filters. In FIG. 1B, a filter made with optical fiber, known as fiber Bragg grating (FBG), is illustrated. In a FBG, the index of refraction of the optical fiber is periodically modified. The period of the modification, d, is related to the center wavelength xcexm of the given filter as xcexm=2 nd/m, where m is the order of the Bragg grating and n is average of the index of refraction of the fiber. Another type of filter frequently used in DWDM systems is a multi-layer interference filter. These filters are constructed with several, sometimes many layers of different optical materials with varying thickness such that a desired transmission (or reflection) curve centered near a predetermined channel center-frequency is obtained as depicted in FIG. 1C.
In the filter approach to DWDM, each data channel is associated with a specific optical filter. The DWDM system therefore consists of many filters, each of which has to be connected or placed in a particular location and/or orientation. A more systematic way to construct a DWDM system is to use wavelength dispersion devices such that many channels can be multiplexed or de-multiplexed with a single device. In FIG. 2A, a device commonly known as an arrayed waveguide grating (AWG) is displayed. As depicted, the AWG can be used to separate all data channels simultaneously. The output channels can be connected directly to individual optical fibers. When using an AWG in the reverse direction, many different signal channels can be combined into a single optical fiber. A prism can also be used to multiplex or de-multiplex optical signals. As displayed in FIG. 2B, due to dispersion, i.e., the index of refraction is different for different frequencies is so that the exit angle is different for channels having different center frequencies. Different output channels are separated in space and connected into individual fibers. Another commonly used device is a diffraction grating, an optical surface which is modified periodically (with a period d) such that when light is directed to this surface, the angle of incidence (xcex1) and diffraction (xcex2) are related to the wavelength of the incoming light xcex according to: d (sin xcex1+sin xcex2)=m xcex, where m is an integer commonly referred to as the order of diffraction. Such a diffraction grating is illustrated in FIG. 2C.
A third type of wavelength separating and combing devices is known as interleavers. FIG. 3A provides a function diagram of an interleaver. These interleavers separate a composite optical signal into two complementary signals in which the odd data channels are branched into one output and the even channels are directed into the other output. In an interleaver application, the frequency space is divided into two parts, 50% for output 1 and 50% for output 2, as illustrated in FIG. 3B. Two typical interleaver devices are depicted in FIG. 3C and FIG. 3D. In FIG. 3C, an interleaver designed based upon a Fabry-Perot etalon is displayed. In this device, two parallel, partially reflecting surfaces are separated by a distance d. The center wavelengths xcexm associated with transmitted channels are given by xcexm=2 nd/m, where m is an integer and n is the index of refraction. In FIG. 3D, an interleaver based on the Michelson interferometer is illustrated. The center wavelengths xcexm associated with channels branched into OUTPUT 2 are given by xcexm=2 n (d1xe2x88x92d2)/m, where m is an integer and n is the index of refraction along the optical path. The performance and optical characteristics of these interleavers can be enhanced with certain modifications. For example, when a partial reflector is inserted into a Michelson interferometer based interleaver, parallel to one of the two mirrors, both the reflection and transmission spectra are significantly improved.
Interleavers provide more flexibility to DWDM system designers and engineers. In FIG. 4, two stages of interleavers are cascaded to provide four outputs each carrying one fourth of the original data channels. The frequency spacing of the adjacent data channels for a particular output is therefore four times the spacing between adjacent data channels in the input signal. Another practical configuration, as demonstrated in FIG. 5, utilizes both the interleaves and wavelength dispersion devices. In this configuration, the optical alignment and/or temperature stability requirements for the dispersion devices are significantly less stringent when the channel spacing is increased to twice the original spacing. In a different configuration as displayed in FIG. 6, an interleaver, or a two-stage cascade of interleavers, is followed by individual filters. In this configuration, filters with a larger channel spacing (e.g., 200 GHz filters) and hence lower tolerance and lower cost can be used to construct state of the art DWDM systems with smaller channel spacing (e.g., 100 GHz or 50 GHz). Therefore, interleavers provide an economical solution for system integrators.
In accordance with the present invention, an interleaver comprises a serial array of optical elements to which a multichannel, continuous spectrum, composite signal is input and two multichannel, non-continuous spectrum, composite signals are output. Two embodiments are disclosed One such embodiment provides output composite signals which are spectrally symmetric in that each such non-continuous spectrum contains the same number of channels albeit of alternating center wavelengths. The other such embodiment provides output composite signals which are spectrally asymmetric in that each such non-continuous spectrum contains either a different number of channels or an equal number of channels of narrower or wider passband.
Each of the preferred embodiments comprises optical elements which split the input composite signal into components of different polarization states, selectively add phase shifts to some of these components and then recombine them a number of times. These optical elements act on the light beams as they make a round trip through the interleavers; i.e., first one way and then the opposite way through the serial arrayed elements. In both of the disclosed embodiments of the invention, the input composite signal and the two output composite signals, are coupled to and exit from the interleaver at the same surface.
The inventive arrays operate on the incident composite signal light by exploiting the splitting, phase-shifting and re-combining of light beams to produce interference effects which attenuate the unwanted wavelength components in each composite output signal and reinforce the desired wavelength components.